It has become increasingly common to use hollow plastic tanks made by a blow molding process as fuel tanks for motor vehicles, watercraft and other applications. In a blow molding process, molten resin material that has been obtained from an extruder is held in a die assembly defining a desired outer profile of the product and blowing air into the molten resin material so as to force the plastic material onto the surface of the die cavity.
Conventionally, the molded product is left in the die assembly for a certain period of time that is required for the molded product to become hard enough to retain its shape. If the molded product is not cooled to an adequately low temperature, it may deform during the subsequent handling. Therefore, conventionally, the product was kept in the die assembly for a significant period of time, and this prevented an efficient utilization of the die assembly.
It has been proposed to immerse the molded tank in water for cooling the same. But the force required for keeping the tank immersed against the buoyant force is so great that it could deform the tank. It has also been proposed to introduce cooling water into the tank. But it was found that removing water out of the tank is time-consuming and not easy to accomplish.